A lithium ion secondary cell has been widely used, as a cell capable of attaining higher voltage/higher energy density than a conventional nickel-cadmium cell or nickel metal hydride cell does, for information-related mobile communication electronic equipment such as mobile phones and laptop personal computers. As a means for solving environmental problems, the application of the lithium ion secondary cell to an onboard use in which the cell is incorporated into an electric vehicle, a hybrid electric vehicle and the like or an industrial use such as an electric power tool is expected to increase in the future.
In the lithium ion secondary cell, a positive electrode active material and a negative electrode active material play an important role in deciding the capacity and output. In a conventional lithium ion secondary cell, lithium cobaltate (LiCoO2) and carbon are often used as the positive electrode active material and the negative electrode active material, respectively. However, with the recent expansion of the application of a lithium ion cell to a hybrid vehicle or an electric vehicle, the cell has been increasingly required to attain not only the enhancement in capacity but also the enhancement in output which indicates the magnitude of the capacity taken out in a short time. In order to make a cell attain highly enhanced output, that is, to efficiently take out a large current from a cell, it is necessary to enhance the electron conductivity and also enhance the ion conductivity of lithium ions at the same time.
On the other hand, for the purpose of making a lithium ion secondary cell attain highly enhanced capacity and highly enhanced output, the search for a next-generation active material has also been actively conducted. In positive electrode active materials, olivine-based materials, that is, active materials such as lithium iron phosphate (LiFePO4) and lithium manganese phosphate (LiMnPO4) have been attracting attention as next-generation active materials. An effect of enhancing the capacity is restrictively exerted because the capacity of lithium iron phosphate or lithium manganese phosphate remains within about 1.2 times the capacity of lithium cobaltate, but lithium iron phosphate and lithium manganese phosphate have a great merit in terms of stable supply since cobalt, which is a rare metal, is not contained therein. Furthermore, in the olivine-based active material, oxygen is hardly emitted therefrom because the oxygen atom is covalently bonded with the phosphorus atom, and the olivine-based active material also has a feature of attaining a high level of safety. Of these, lithium manganese phosphate can be expected to also contribute to the enhancement in output because the discharge potential is high in the case of being used as the positive electrode active material of a lithium ion secondary cell. However, unlike lithium cobaltate (LiCoO2) or the like, the olivine-based positive electrode active material has a problem that it is difficult to take out the capacity inherently possessed by the active material, that is, the theoretical capacity, because the change in the crystal lattice associated with the charge-discharge is significant, and the olivine-based positive electrode active material is low in electron conductivity and ion conductivity.
On that account, an olivine-based positive electrode material is micronized so as to have a crystallite size of about 200 nm, and furthermore, the particle surface is coated with carbon to achieve the reduction of the influence of a strain associated with the change in the crystal lattice size and the enhancement in ion conductivity and electron conductivity. Although the theoretical capacity is substantially exhibited by this method with regard to lithium iron phosphate, with regard to lithium manganese phosphate, it is difficult to attain highly enhanced capacity of a cell only by this method, and thus, there have been reported various attempts aimed at making lithium manganese phosphate exhibit its theoretical capacity.
It has been well known that the shape of a particle is of importance for making a cell prepared with lithium manganese phosphate attain highly enhanced capacity. Lithium manganese phosphate, which is extremely low in ion conductivity and electron conductivity, is required to have a smaller particle diameter than that of lithium iron phosphate. Lithium manganese phosphate is also required to have a shape with which the Li-ion conductivity is enhanced and the influence of a strain associated with the charge-discharge reaction is reduced.
In order to attain such a shape, there have been proposed plate-like particles oriented in the b-axis direction. This is an idea of making the moving distance of a lithium ion in the inside of a particle as short as possible and widening the area of an opening through which a lithium ion is extracted and inserted since the lithium ion can move only in the b-axis direction in lithium manganese phosphate. For example, by each of manufacturing methods disclosed in Patent Document 1 and Non-Patent Document 1, a kind of lithium manganese phosphate which is oriented along the b-axis in an aqueous diethylene glycol solution and has a thickness of about 20 to 30 nm is obtained. Moreover, Patent Document 2 also discloses an effect of lithium manganese phosphate oriented in the b-axis direction.